The present invention relates to a system for stabilizing the output wavelength of a laser oscillator that generates light for transmission in an optical communication network, a system for monitoring the wavelength of a light signal received from an optical communication network, and more generally a method of monitoring the wavelength of a modulated light signal.
As optical communication networks carry increasing amounts of information, there is a trend toward the use of increasingly narrow optical wavelength bands, to avoid running out of wavelength resources. This trend has made it essential to study techniques for stabilizing the output wavelengths of optical signal transmitters.
In particular, growing communication traffic has led to the use of wavelength-division multiplexing systems with increasing numbers of wavelengths, therefore with increasingly narrow wavelength spacing. If the stability of the individual wavelengths deteriorates, crosstalk occurs between adjacent wavelengths, and communication quality is degraded. Communication systems in which wavelength-division multiplexing is employed therefore have stringent requirements for wavelength stability.
The transmitters used in optical communication systems, however, generally employ semiconductor lasers having a temperature-dependent wavelength characteristic; the emitted wavelength varies with the temperature of the laser. In conventional transmitters, this temperature-dependency problem is dealt with by measuring the temperature of the semiconductor laser, and using a heat-pumping device such as a Peltier device to hold the semiconductor laser at a substantially constant temperature. Conventional optical communication networks rely on this scheme to maintain wavelength stability, and do not attempt to monitor the wavelength of the received optical signals.
FIG. 7 shows a conventional optical transmitting apparatus that transmits a signal on, for example, an optical fiber in an optical communication network. The optical transmitting apparatus 7 includes a transmitting light source 41 that generates coherent output light OL with a fixed frequency, a transmitting modulator 42 that modulates the fixed-frequency output light OL to obtain a transmit light signal SL, and a control unit 96 that receives temperature information from the transmitting light source 41, supplies the transmitting light source 41 with control signals for adjusting the power and wavelength of the laser output light OL, receives information to be transmitted (send data, SD) from an external source (not visible), and sends a corresponding modulating signal MD to the transmitting modulator 42.
The transmitting light source 41 includes a laser oscillator 51 such as a semiconductor laser, a wavelength adjustment unit 52 including a heat-pumping device such as a Peltier device, and a temperature-measuring unit 53 including a device such as a thermistor that detects ambient temperature changes as changes in electrical resistance.
The conventional optical transmitting apparatus 7 operates as follows.
When information SD to be transmitted is input, the control unit 96 controls the transmitting light source 41 so that the laser oscillator 51 outputs light OL of a fixed wavelength, and supplies the transmitting modulator 42 with a modulating signal MD. The transmitting modulator 42 modulates the output light OL according to the modulating signal MD to obtain the transmit light signal SL.
If information SD to be transmitted is input continuously, the laser oscillator 51 operates continuously, and its temperature begins to rise. The temperature of the laser oscillator 51 may also rise because of heat generated from another device (not visible) in the equipment, or because of a rise in the ambient temperature. Similarly, a drop in the ambient temperature may lower the temperature of the laser oscillator 51. If the temperature of the laser oscillator 51 varies, so does the wavelength of the output light OL.
To limit these wavelength variations, the temperature-measuring unit 53 detects the temperature in the vicinity of the laser oscillator 51, and the control unit 96 responds by controlling the wavelength adjustment unit 52 so as to keep the temperature around the laser oscillator 51 within a fixed range. For example, the control unit 96 may operate according to two thresholds, controlling the wavelength adjustment unit 52 so as to lower the temperature around the laser oscillator 51 if the temperature indicated by the temperature-measuring unit 53 exceeds the upper threshold, and to raise the temperature around the laser oscillator 51 if the temperature indicated by the temperature-measuring unit 53 falls below the lower threshold. A feedback loop is thereby established, involving the wavelength adjustment unit 52, the temperature-measuring unit 53, and the control unit 96.
A problem with this feedback loop is that it is not always possible to mount the thermistor or other temperature-sensing element of the temperature-measuring unit 53 close enough to the laser oscillator 51 to detect its temperature accurately. There may be a considerable difference between the temperature measured by the temperature-measuring unit 53 and the actual temperature of the laser oscillator 51, preventing the control unit 96 from keeping the temperature of the laser oscillator 51 within the desired range. Since the feedback loop does not include any measurement of the wavelength of the output light OL or light signal SL, there is no guarantee that feedback control will actually produce the desired wavelength.
An object of the present invention is to provide a method of monitoring the wavelength of a modulated light signal.
Another object of the invention is to stabilize the wavelength of light generated by a laser oscillator and modulated for transmission in an optical communication system.
Another object is to monitor the wavelength of a modulated light signal received in an optical communication system.
The invented method of monitoring the wavelength of a modulated light signal includes the steps of generating criterion light having a stable wavelength, splitting the modulated light signal into at least two parts, and combining one part of the modulated light signal with the criterion light, thereby obtaining a combined light signal. The combined light signal is compared with the modulating signal, and an error rate indicating how often the combined light signal disagrees with the modulating signal is calculated. A high error rate indicates wavelength agreement between the modulated light signal and the criterion signal, since wavelength agreement leads to interference when the modulated light signal and criterion light are combined.
The comparison step may include conversion of the combined light signal to an electrical signal.
The criterion light may also be modulated according to the modulating signal, to make the error rate a more sensitive indicator of wavelength agreement.
The polarization planes of the modulated light signal and the criterion light are preferably controlled so that the modulated light signal and criterion light are polarized in the same plane when combined, leading to greater interference when their wavelengths match.
In an optical transmitting apparatus, the invented method can be used to control the wavelength of light output by a laser oscillator. For example, the wavelength can be controlled by controlling the temperature of the laser oscillator according to the error rate. In this case, the modulating signal is a signal by which the output light of the laser oscillator is modulated.
The invention may also be used to monitor the wavelength of a modulated light signal received by an optical receiving apparatus. In this case, the modulating signal is determined from the output of the optical receiving apparatus.
When the optical transmitting apparatus or the optical receiving apparatus is located at a node in an optical communication network, the reference light source may be external to the node, and may supply the criterion light to a plurality of nodes in the network.